First, the federal government does not have enough boots on the ground to fight everyone at once. If a couple of small time war lords show up, they will bury them in lead to make an example of them. I truly believe that the first dozen or so will be put down fast. However as more show up at once, in non-descript places and in cities, they will focus on the big city boys to have the most impact with somewhat limited resources.
Now, in the small time rural areas, there will be county sheriffs that will seek to keep the peace. They will also have plenty of help, but not enough against a large band of toughs and a war lord. Rural areas have a lot of in fighting and they only band together when it is clear that there is a danger to everyone. Depending on the area, this wait and see attitude will allow war lords to become entrenched. War lords will have more resources and out gun the sheriff IF the locals can not end in fighting.
When was the last time you went without electricity, running water, food, and had babies screaming for food...now you know why I prep. These are the things a mother's nightmares are made of.